Fuel control system for gas turbine engine

ABSTRACT

A fuel control system for a gas turbine engine includes a metering device and a servo-operated control valve through which fuel flows from the metering device to the engine. A pilot valve for the servo-operated control valve is responsive to engine speed and is also operable, when a metering device control member moves past a predetermined position, to cause the servo-operated control valve to reduce fuel flow.

This is a continuation of application Ser. No. 485,881, filed July 5,1974.

This invention relates to fuel control systems for gas turbine engines.

According to the invention a fuel control system for a gas turbineengine comprises a pump, a variable metering device downstream of saidpump, a control valve arrangement downstream of said variable meteringarrangement, said control valve arrangement having an inletcommunicating with said variable metering arrangement and first andsecond oulets respectively communicating with burners of the engine andwith an upstream side of said pump and a control element movable inresponse to an increase in a servo pressure signal to open said firstoutlet and shut said second outlet, and a pilot valve responsive to anincrease in engine speed to reduce said servo pressure signal.

Examples of the invention will now be described with reference to theaccompanying drawings, in which:

FIGS. 1 to 3 show diagrammatically the several parts of a fuel controlsystem, and should be read in conjunction, and

FIG. 4 shows an alternative form of fuel control system.

FIGS. 1 to 3 show a fuel control arrangement for a three-spool gastubine engine 10, that is, an engine having three compressors and threeassociated turbines being carried on concentric shafts.

A centrifugal pump 11 supplies fuel to a positive-displacement pump 12driven by the engine 10. A spring-loaded pressure relief valve 13 isconnected across the pump 12.

Downstream of pump 12 is a variable metering arrangement 14 whichcomprises a sleeve 15a slidable in a body 16 relative to a fixed sleeve15b. Slidable within sleeves 15a, 15b is a further sleeve 17. Sleeve 17has triangular ports 18 which can move relative to a gap 19 definedbetween sleeves 15a, 15b to provide a variable metering orifice.

Sleeve 15a is movable in a direction relative to sleeve 17, to reducefuel flow by a flyweight arrangement 20 driven by the high speed shaftof engine 10. Engaged with sleeve 15a is a lever 21, which, as shown inFIG. 1, is mounted for movement about a fixed axis 22 and has an arm 23formed on one edge with a cam surface 24. A cam element 25 has a pair ofcam faces 26, 27 which are formed along opposite sides of element 25. Apair of support rollers 28, 29 engage the cam face 27 and the positionsof rollers 28, 29 are adjustable by threaded stems.

A transmission device 30 comprises a carrier 31 upon which a pair ofarms 32, 33 are pivotally mounted. Bearing rollers 34, 35 on the ends ofrespective arms 32, 33 engage cam surfaces 24 and 26. Rollers 34, 35 arebiased outwardly against the respective cam surfaces by a spring 36, thearrangement being such that spring 36 acts, through lever 21 to biassleeve 15 against movement by flyweight arrangement 20.

Carrier 31 is mounted for pivotal movement about a fixed axis betweenthe cam surfaces 24, 26 so that movement of the force of spring 36 aboutpivot 22, and thus the bias applied to sleeve 15a, is variable. Carrier31 is movable by a cam 37 which is in turn responsive to a lever 38movable from a central, idle, position shown in FIG. 1, in eitherdirection to demand forward or reverse thrust from the engine 10. Cam 37is formed so that the power demand, for similar engine movement from theidle position, is the same for both forward and reverse thrust. It willbe understood that reverse thrust is obtained from engine 10 bydeflectors (not shown) operated in rsponse to movement of lever 38.

Cam element 25 is freely pivoted on a link 39 which is in turn freelypivoted on a lever 40 movable about a fixed axis 41. Lever 40 is biasedby a spring 42 towards an edge cam 43 rotated by a stepper motor 44.Motor 44 is controlled by output pulses from a control circuit 45.Circuit 45 is a digital circuit responsive to the speed N_(H) of thehigh speed shaft of engine 10, to the temperature T₁ at the enginecompressor inlet, to the Mach No. MN and altitude of an aircraft inwhich the engine 10 is mounted, to the exhaust temperature T₇ of theengine combustion chamber, to the position Θ of lever 38, and also tothe setting of a selector switch 46 to drive motor 44 to rotate cam 43so as to move cam element 25 upwardly, as seen in FIG. 1. Upwardmovement of cam element 25 rotates the latter anticlockwise about itspivotal connection with link 39 and thus increases the bias of spring 36on sleeve 15a, for a given position of carrier 31. Selector switch 46 ismanually operable to correspond to required operating modes of anaircraft in which the engine 10 is mounted, as for example take-off,maximum climb, and maximum cruise.

In the event of failure of circuit 44 the cam 43 can be renderedineffective by means of a further cam 47. Cam 47 is rotatable to liftlever 40 away from cam 43 by a selector lever 48 when the latter ismoved to the position indicated at A in FIG. 3.

Referring to FIG. 2 the sleeve 17 of variable metering arrangement 14 ismovable by a lever 50 which is coupled to a bellows arrangement having apair of bellows units 51, 52 arranged in tandem. Unit 51 is evacuatedand unit 52 is subjected internally to an intermediate pressure P₃ fromthe engine compressor. Both units 51, 52 are subjected externally to apressure P_(4P) which is derived by means of a fluid potentiometercomrising a restrictor 53 and a venturi 54 connected in series betweenpressure P₃ and a pressure P₄ derived from the output of the finalcompressor stage of the engine.

In use, therefore, variable metering arrangement 14 is responsive to anincrease in pressure P_(4P), or to a decrease in pressure P₃, toincrease fuel flow. Arrangement 14 is also responsive to an increase inthe speed N_(H) of the high speed shaft of the engine to decrease fuelflow, and the sleeve 15a is biased against movement by flyweightarrangement 20, by spring 36, acting through lever 21. The bias appliedby spring 36 is increased by movement of lever 38 in either directionaway from its central, idling, position. Movement of sleeve 15a islimited by adjustable stops 55, 56 which effectively act to limitacceleration and deceleration respectively.

Movement of sleeve 17 in a direction to decrease fuel flow is limited bya stop 57. Stop 57 is movable by a cam 58 rotatable by selector lever48. Stop 57 is movable between a first position, shown in FIG. 2 andselected by a position B of lever 48, and a second position selected bypositions C or D of lever 48. Position B of lever 48 corresponds tonormal running of the engine 10, and positions C and D to "fuel rich"and "fuel lean" engine starting conditions respectively. Stop 57provides, for a given engine speed as manifested by the position ofsleeve 15, a minimum fuel flow through the metering arrangement 14.

Extending axially through sleeve 17 is a stem 60 which terminates in apiston element 61 slidable in a ported sleeve 62. The ends of sleeve 62communicate respectively with the upstream and downstream sides ofvariable metering arrangement 14. Sleeve 62 has ports 63 whichcommunicate via a passage 64 with the upstream side of pump 12. Element61 thus provides a spill valve for metering arrangement 14.

Slidable on stem 60 adjacent piston element 61 is a further pistonelement 65. Sliding movement of element 65 is restricted in bothdirections by a projection 66 on stem 60. Element 65 co-operates withfurther ports 67 in sleeve 62 to define a throttle valve in series withmetering arrangement 14. Ports 67 communicate via a passage 68 with afurther valve arrangement 69, later to be described. Element 65 is urgedin a direction to uncover ports 67 by the pressure downstream ofmetering arrangement 14, and in the opposite direction by a spring 70which also engages piston element 61. Piston element 65 has a relievedportion 65a so that the space between elements 61, 65 is alwayssubjected to the pressure in passage 68.

Stem 60 is engaged by a further set of governor flyweights 71 which arealso driven at the speed N_(H) of the engine high speed shaft.Flyweights 71 are biased by a spring 72 against movement in response toan increase in speed N_(H). Spring 72 thus sets a minimum force on stem60 and, by urging the latter towards a closed position, a minimumpressure drop across the variable metering arrangement 14. An adequateflow level at light-up is thus provided. This light-up flow level isresponsive to altitude by virtue of movement of sleeve 17 by bellows 51,52.

The downstream side of metering arrangement 14 communicates via apassage 73 and a shut-off cock 74 with pilot burners of the engine 10.Cock 74 is operated by selector 48 so as to be open in all positions oflever 48 except E, which position corresponds to engine shut-down.

Piston element 61 is biased against the delivery pressure of pump 12,and in a direction to reduce spill flow and permit element 65 to openthe throttle valve, by a spring 75. The force exerted by spring 75 isvariable by a cam 76 which is rotatable by selector lever 48 so that inposition C of lever 48 (corresponding to fuel-rich starting) the springforce is increased and the spill flow reduced.

During starting, the pressure downstream of metering arrangement is low,and spring 70 overcomes this pressure to urge piston element 65 to shutthe throttle valve. Substantially the whole of the fuel delivered bypump 12 is thus supplied via passage 73 to the engine pilot burners. Atlow levels of fuel flow through metering arrangement 14, the throttlevalve thus acts as a pressurising valve to maintain the pressure inpassage 73 above that in passage 68 by an amount which does not fallbelow a minimum value set of spring 70.

When engine speed rises to the level at which flyweights 71 overcomespring 72, the fuel flow through arrangement 14 increases and the fueldelivery of pump 12 also increases. The pressures upstream anddownstream of metering arrangement 14 increase. The downstream pressureovercomes the force exerted by spring 70 and element 61 engages element65. These elements substantially move as a unit and are responsive tothe difference in pressures across metering arrangement 14. Elements 61,65 are also responsive, via flyweights 71, to the speed N_(H) of theengine. In these conditions, therefore, elements 61, 65 act to maintainthe pressure difference across metering arrangement 14 substantiallyconstant for any given speed N_(H).

The valve 69 (FIG. 3) between passages 68 and the engine main burnershas a pair of outlets 77, 78 which communicate respectively with passage64 and with the main burners. A spool control element 79 is movable soas to divide the flow from passage 68 between outlets 77, 78. Element 79is responsive to the pressure in passage 68 and is formed with lands ofdifferent diameters so as to be biased, by the pressure in passage 68,in a direction to increase flow through outlet 77 and decrease flowthrough outlet 78. Element 79 is also biased in the same direction by aspring 80 and by a high pressure signal applied via a line 81 from theoutlet of pump 12. Element 79 is urged in the opposite direction by aservo pressure signal in a chamber 82, derived from the pressure in line81 via a restrictor 83. Chamber 82 communicates via a port 84, a passage85 and a pilot valve arrangement 86 with passage 68. Port 84 is placedso as to form a valve, in co-operation with the larger end of spoolelement 79. A condition thus can exist in which, with port 84 partly,shut, the forces on element 79 are in equilibrium. In this equilibriumcondition port 78 remains at least partly open, so that the main burnersare not extinguished.

Pilot valve 86 is operated by a torque motor 87 responsive to anelectronic control circuit 88. Circuit 88 receives signals dependent onthe speeds N_(L), N_(I) of the low-speed and intermediate-speed engineshafts and also on temperature T₇, so that an increase beyondpredetermined levels of these values opens pilot valve 86 to reduce theservo pressure in chamber 82 and thereby reduce fuel flow to the enginemain burners.

A lever 89 has an adjustably-positioned pivot 90 and is engageable bothwith the control element of pilot valve 86 and also with the lever 50 ofthe variable metering arrangement 14 when the lever 50 has been moved bymore than a predetermined amount in response to pressures P₃ and P_(4P).Lever 89 thus acts to reduce fuel flow to the engine in order to limitthe difference between pressures P₃ and P_(4P), and hence to determine amaximum compressor delivery pressure.

Chamber 82 of valve 69 also communicates via a passage 91 and a shut-offcock 92 with passage 68. Shut-off cock 92 is operated by selector lever48 so as to be opened only when the latter is in position E, i.e. whenthe engine is shut down. With cock 92 open, servo pressure in chamber 82falls to cause outlet 78 to be shut off completely and all fuel flow tovalve 69 to be returned to the upstream side of pump 12.

The system shown in FIG. 4 is a modification of that described above,similar parts having identical reference numbers

A three-spool gas turbine engine 10 is supplied with fuel from apositive-displacement pump 12 driven by the engine 10. Downstream ofpump 12 is a variable metering arrangement 14.

Arrangement 14 includes, as before, a control element 17 which ismovable in first and second directions respectively to increase andreduce fuel flow. This control element 17 is movable in said firstdirection in response to a decrease in pressure P₃ or to an increase ina pressure P_(4P) derived from a fluid potentiometer connected betweenpressure P₃ and P₄. Arrangement 14 also includes, as before, a sleeve15a which is responsive to speed N_(H) and to the position O of a powerdemand lever 38. The response of sleeve 15a to these parameters can betrimmed, as previously described with reference to FIGS. 1-3 inaccordance with electrical signals from a control circuit 45 responsiveto speed N_(H), temperature T₁, Mach. No. MN, aircraft altitude andcombustion exhaust temperature T₇.

Downstream of metering arrangement 14 is a valve arrangement 100, whichis a modification of the valve arrangement 69 described with referenceto FIGS. 1 to 3.

Valve 100 lies between an outlet passage 68 of metering arrangement 14and the main burners of the engine 10. Valve 100 has a pair of outlets102, 103 which communicate respectively with a passage 64 and with theengine main burners. Passage 64 communicates with the upstream side ofpump 12.

A spool control element 101 is movable so as to divide the flow frompassage 68 between outlets 102, 103. Element 101 is biased, by thepressure downstream of pump 12 and applied via passage 81, and by aspring 104, in a direction to increase flow through outlet 102 anddecreases flow through outlet 103. Element 101 is urged in the oppositedirection by a servo pressure signal in a chamber 105, derived from thepressure in line 81 via a restrictor 106. The area of element 101subjected to the pressure in chamber 105 is greater than that subjectedto the pressure in passage 81. Chamber 105 communicates via a port 107,a passage 108 and a pilot valve arrangement 109 with passage 68. Port107 is placed so as to form a valve, in co-operation with a land onspool element 101. A condition thus can exist in which, with port 107partly shut, the forces on element 101 are in equilibrium. In thisequilibrium condition port 103 remains at least partly open, so that themain burners are not extinguished.

Pilot valve 109 is operated by a torque motor 87 responsive to anelectronic control circuit 88. Circuit 88 receives signals dependent onthe speeds N_(L), N_(I) of the low-speed and intermediate-speed engineshafts and also on temperature T₇, so that an increase beyondpredetermined levels of these values opens pilot valve 109 to reduce theservo pressure in chamber 105 and thereby reduce fuel flow to the enginemain burners. A spring 111 is engaged between element 101 and thecontrol element 112 of pilot valve 109. The arrangement is such thatmovement of element 112 to shut valve 109, and thereby to increasepressure in chamber 105, is opposed by the resultant movement of spoolcontrol element 101. Control element 112 thus adopts an equilibriumposition in which the force applied by torque motor 87 is balanced bythe net force exerted by spring 111 and by an opposing spring 114. Theforce exerted by spring 111 is dependent on the position of element 101.Element 101 thus adopts a position dependent on the current supplied totorque motor 87.

A lever 113 moves with the control element 112 of pilot valve 109 and isalso movable by the aforesaid control element 17 in metering arrangement14 when element 17 moves by more than a predetermined amount in theaforesaid first direction to increase fuel flow. Lever 113 can thus act,via pilot valve 109, to reduce fuel flow to the engine in order to limitthe difference between pressures P₃ and P_(4P), and hence to determinemaximum compressor delivery pressure.

Chamber 105 of valve 100 also communicates via a passage 90 and ashut-off cock 91 with passage 68. Shut-off cock 91 is operated byselector lever 48 so as to be opened only when the engine is shut down.With cock 91 open, servo pressure in chamber 105 falls to cause outlet103 to be shut off completely and all fuel flow to valve 100 to bereturned to the upstream side of pump 12 via outlet 102.

I claim:
 1. A fuel control system for a gas turbine engine, comprising apump, a variable metering device downstream of said pump, a controlvalve downstream of said variable metering device, said variablemetering device including a control member movable in response to enginespeed to vary fuel flow through said device, said control valve havingan inlet communicating with said variable metering device and first andsecond outlets respectively communicating with burners of the engine andwith an upstream side of said pump, said control valve having a controlelement movable in response to an increase in a servo pressure signal toopen said first outlet and shut said second outlet, a pilot valveresponsive to an increase in engine speed to reduce said servo pressuresignal, and a limiting valve which is progressively shut by said controlvalve control element as the latter moves to shut said first outlet,said limiting valve being in series with said pilot valve wherebyreduction of said servo pressure signal by said pilot valve is limitedby said limiting valve.
 2. A system as claimed in claim 1 which includesmeans responsive to movement, beyond a predetermined limit, of saidmetering device control member in a direction to increase fuel flow, forcausing said pilot valve to reduce said servo pressure signal.
 3. Asystem as claimed in claim 2 in which said means responsive to controlmember movement comprises a lever coacting with said control member andwith said pilot valve.
 4. A system as claimed in claim 3 in which theposition of the fulcrum of said lever is adjustable.
 5. A system asclaimed in claim 3 which includes a control circuit responsive to enginespeed to produce an electrical control signal, an actuator responsive tosaid control signal, and a control element for said pilot valve movableby said actuator.
 6. A system as claimed in claim 5 which includes aspring interconnecting said pilot valve control element and said controlvalve control element, such that movement of the latter in a directionto open said first outlet urges said pilot valve control element in adirection to decrease said servo pressure signal.
 7. A system as claimedin claim 5 in which a pivotal mounting for said pilot valve controlelement also forms a fulcrum for said lever.
 8. A system as claimed inclaim 1 which includes a further valve, in parallel with said pilotvalve and operable when the engine is shut down, for reducing said servopressure signal.